WebPPicture: clarify the ownership of user-owned data.

It's explicitly safe (and recommended!) to plug external data into
the pic->y/u/v/argb fields. They are guaranteed to be preserved
by the encoding process if no conversion is needed.

Change-Id: I325ca41a6a834f7f028431c605dddef67e9542cc

src/webp/encode.h

@@ -292,6 +292,11 @@ typedef enum WebPEncodingError {
 #define WEBP_MAX_DIMENSION 16383
 
 // Main exchange structure (input samples, output bytes, statistics)
+//
+// Once WebPPictureInit() has been called, it's ok to make all the INPUT fields
+// (use_argb, y/u/v, argb, ...) point to user-owned data, even if
+// WebPPictureAlloc() has been called. Depending on the value use_argb,
+// it's guaranteed that either *argb or *y/*u/*v content will be kept untouched.
 struct WebPPicture {
   //   INPUT
   //////////////